FIG. 1(a) illustrates a well know Bragg grating device 5 formed from silica based optical waveguides. One important application of an integrated optical Bragg grating is the add-drop filter. In performing the add-drop filter function, the waveguides in Bragg grating 5 include a port 1 which receives initial traffic carrying several wavelengths. The dropped wavelength is Bragg reflected and leaves through port 2. The remaining wavelengths pass through the corrugations and leave through port 4. An added wavelength enters port 3, is Bragg reflected and leaves through port 4. FIGS. 1(b) and 1(c) illustrate the bands of dropped and passed signals R.sub.x and T.sub.x, respectively.
Other devices which provide optical interconnections accomplish optical wavelength multiplexing and demultiplexing using a polarity of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates through an optical grating comprising a series of optical waveguides, wherein each of the waveguides differs in length with respect to its nearest neighbor by a predetermined amount. The grating is connected to the input of a second star coupler and the outputs of which form outputs of a switching, multiplexing, and demultiplexing apparatus. Examples of such interconnection apparatuses are disclosed in U.S. Pat. Nos. 5,002,350 and 5,136,671, which are expressly incorporated herein by reference.
The geometry of the above-referenced devices may be such that a polarity of separate and distinct wavelengths each launch into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, a polarity of input wavelengths is then separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelengths also permits the switching between any selected input port to any selected output port. Accordingly, these devices are generally referred to as frequency routing devices and, more specifically, wavelength division multiplexers (WDMs). Ideally, the individual wavelength channel position of the WDMs and the associated transmitters should be aligned to a predefined, industry-established wavelength grid referred herein as .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 . . . .lambda..sub.n.
Unfortunately, however, in practice the wavelengths or center frequencies of both the transmitter and WDM channels drift with time and/or have initial fabrication errors. Such drifting or errors each result in wavelengths of the respective optical devices to not be aligned as desired and thus adversely affect the operation of that device within a communication system. Additionally, ambient temperature affects both the path length and refractive index of WDMs, producing shifts of center frequency of WDM passbands with changes in ambient temperature. To stabilize center frequencies associated with conventional WDM passbands, a temperature controller must be used to stabilize the substrate temperature. The climate control devices require power for operation, thus limiting the use of WDMs to areas where power is accessible, such as central telephone switching offices. Furthermore, the reliability of the heaters and temperature sensors are of concern to some customers who require redundant components to be present to avoid any failure during the lifetime of the device. Moreover, where power is not available to deliver climate control, WDMs may only be configured to have passbands which can tolerate center frequency drifting, thus limiting the number of channels for multiplexing or switch, and increasing the number of WDMs and cost associated with the required signal processing capability.
To date, devices have typically used what may be referred to as a "set and forget" scheme. In other words, existing devices have simply relied on the passband width of the WDM and/or transmitters being lirge enough to tolerate any and all of the wavelengths inaccuracies that may be present due to at least the reasons set above. In such a system, the WDM passband requires a large channel space, therefore significantly limiting the number of channels.
Furthermore, to be effectively used in an increasingly demanding optical communication system of today where WDMs are going to smaller channel spacings, i.e., less than about 50 GHz, and large channel counts, i.e., greater than or equal to about 32 channels, improvement is needed in the ability to provide accurate center wavelength control in a WDM system and integrated device.
With the forgoing disadvantages of prior art optical interconnection devices in mind, it is an object of the present invention to provide an optical transmission device having an optical planar waveguide with temperature independent transmission properties.
It is another object of the present invention to achieve passive control over thermal effects on planar optical waveguidles using non-planar geometric distortion of a substrate which supports the optical waveguides.
Another object of the present invention to eliminate the need for temperature control devices to prevent the shifting of center frequencies for waveguides and optical transmission devices.
Another object of the present invention is to provide WDMs which have small channel spacings having temperature independent center frequencies.
Other objects, features and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.